1. Field of the Invention
The present invention relates to grapple saws, and more particularly, to a grapple saw that maintains a positive clamping pressure on the grapple arms during the cutting operation.
2. Description of Related Art
Grapple saws have become widely accepted as providing a fast and efficient means for harvesting trees. A grapple saw is typically attached to a mobile unit, such as a tractor, which supports the grapple saw and allows it to be maneuvered during the tree harvesting operation. In operation, the mobile unit is driven up to a tree to be harvested and the grapple saw is moved into position adjacent the tree. In a typical grapple saw, two hydraulic grapple arms are provided. With the saw in position adjacent the tree, pressure is applied to a hydraulic piston controlling the grapple arms causing them pivot and clamp the tree against the frame of the grapple saw. The grapple arms are then locked in the clamped position where they serve to grip the tree and support it during the cutting operation.
The base of the grapple saw is provided with a chain saw blade for severing the tree. Normally the chain saw blade is in a retracted position within the grapple saw housing. However, with the grapple arms in the clamped position, the saw swings out of the housing to sever the base of the tree. As the tree is being severed, the grapple saw frame can apply directional pressure to the tree. This directional pressure is important to maintain an open kerf and prevent binding of the chain saw blade. The directional pressure can also be used to control the direction the severed tree will fall.
In order to maintain the directional pressure on the tree during felling, it is necessary for the grapple arms to maintain a firm grip on the tree. However, in existing grapple saws once the grapple arms are moved to the clamped position, no further pressure is applied to the hydraulic cylinder controlling the grapple arms. That is, the clamping pressure is maintained only by the residual pressure in the hydraulic cylinder and the hydraulic lines leading thereto. As a result, it is not uncommon for the grapple arms to loosen during the felling operation. Such loosening can result from natural leakage in the hydraulic circuit or from a shift in the grapple saw frame on the tree during the felling operation.
Whatever the cause, loosening of the grapple arms may allow the grapple saw frame to slide and shift relative to the tree. If this occurs while the saw blade is in the kerf, the saw blade can be bent or broken, dramatically reducing the efficiency of tree harvesting operations.